Lecture 2B. Translocation

Translocation

the movement of assimilates (sugars and other chemicals) from the leaf through the phloem to other areas for storage, utilization, and consumption by the plant

so that cells within the plant tissues can receive the nutrients they need for cell processes

Why need a transport system in plants?

Sugar

roots can obtain water, but not

sugar, air

leaves can produce ____ but cannot

get water from the ___

metabolism

sugars are required for ______ all the time, in all tissues

source tissues, during the day

sugars are produced only by

from source to sink over short term

 from storage tissues to young tissues over long term

translocation of sugars occurs

Sucrose is a disaccharide

glucose + fructose

Sucrose

the transport sugar in plants

the principal photosynthetic product for distribution to different plant organs

glucose, sucrose

_____, as an initial product of photosynthesis, is converted to ____ before transport

storage sugar

Sucrose is an important ______

carbon

Sucrose is a major form for translocation of _____

 from photosynthetic organs (mostly leaves)

 in germinating seeds after starch or lipid breakdown

“From source to sink”

Direction of translocation

Source

source of sugars (e.g. leaf, storage organs); a plant part that releases sucrose to the phloem

Sink

receiver of sugars (e.g. root, fruit); a plant part that removes sucrose from the phloem (e.g. root)

Phloem loading

Short-distance pre-phloem transport

Long-distance transport

Transport (or path) phloem

Short-distance post-phloem transport

Release phloem

Allocation

-the channeling of fixed carbon into various metabolic pathways within an organ or tissue

The balance among metabolic processes within source cells determines how much photosynthate is available for export to other parts of the plant.

Partitioning

-distribution of assimilates to competing sinks

-A plant's sinks compete for a share of the available photosynthate. Allocation within sink cells can influence sink strength.

 metabolic utilization within the chloroplast

 synthesis of starch within the chloroplasts

 synthesis of sucrose for export to sink

Allocation in source organ

 metabolic utilization and growth processes

 storage

Allocation in sink organ

the roots

In partitioning… lower mature leaves feed mainly ____

the young leaves and shoot apex

In partitioning… higher mature leaves feed mainly __________

Source leaves

 preferentially supply sink organs with which they have a vascular connection

 e.g. flower or fruit nearest to them (directly above or below them)

- basis for flower or fruit thinning

Phloem

transports sugars and other items. In angiosperms, sieve-tube elements contain the sugar solution. Sieve-tube cells are surrounded by various support cells.

Mechanism for Phloem Transport

Mass or bulk flow (Münch pressure-flow model)

Mass or bulk flow (Münch pressure-flow model)

the primary mechanism for long-distance transport of sugars in the

phloem

osmotic pressure gradients

Mass or bulk flow is driven by _________ between the source and the sink

high osmotic pressure

In mass bulk or flow, sugars actively loaded into sieve tube elements create ________, drawing water in and generating bulk flow toward sink tissues where sugars are unloaded

1.) Simple diffusion

2.) Facilitated diffusion

3.) Active transport across membranes

Phloem Loading and Unloading: Short-distance transport mechanisms

1.) Symplastic pathway (via plasmodesmata)

2.) Apoplastic pathway (via cell wall)

Pathways of phloem loading and unloading

Simple diffusion

passive movement of sugars over short distances (e.g. from mesophyll cells to companion cells)  molecules move from an area of high concentration to an area of low concentration

Facilitated diffusion

 passive movement of sugars across membranes via specific carrier proteins

 functions in phloem unloading at sink tissues

Active transport across membranes

 involves proton-coupled sucrose transporters (e.g. A. thaliana SUC4) that move sugars against their concentration gradient.

 essential for phloem loading and unloading.

plasmodesmata

In symplastic transport, sugars move through the _______

Symplastic Transport

 sugars move through the plasmodesmata

(connections between adjacent cells)

 transport occur via regulated passive diffusion

Apoplastic Transport

 sugars exit (cell wall space) and are actively taken up into sieve

elements and companion cells

 requires active transport via sucrose transporters (SUTs) and monosaccharide transporters (MSTs) in the companion cells

Implications on Crop Production

Maximizing Yield, Enhancing sink strength

Grain filling in rice and wheat

Maximizing Yield: efficient translocation of sugars from leaves to grains (known as “remobilization”) increases grain filling and yield. Late season nitrogen fertilizer application can enhance this process.

Sucrose content in sugarcane

Maximizing Yield: translocation of sucrose to stalks determines

sugar yield; selection of varieties with efficient phloem loading

Pruning

Enhancing sink strength: ______ lower leaves increases sugar translocation to fruits, improving size and sweetness.

phloem loading and unloading

Active transport across membranes is essential for